U.S. Pat. No. 5,328,475 (hereinafter called prior art) granted to the same inventor of this application discloses a safety syringe in which the needle, as coupled with the plunger after finishing an injection, will be retracted into a syringe cylinder and will be automatically inclined to prevent its outward re-protrusion for safety purpose.
The shank portion 22 of the needle device 2 of the prior art is generally formed as a cylindrical shape and has a packing ring 26 fastened on the shank portion and engaged with a ring groove 120 recessed in the sleeve portion 12 of the syringe means 1.
When pushing the plunger 31 forwardly for performing an injection, the needle 2 may be accidentally ejected outwardly as subjected to the pressure of the boosted liquid medicine 4 within the syringe cylinder.
Therefore, the present inventor has invented a mechanism for preventing an unexpected ejection of the needle from the syringe during the injection as described hereinafter.